Cryogenic refrigeration systems are replacing mechanical systems for transporting frozen cargo, particularly by railcar. A particularly effective system for transporting cargo at frozen temperatures is disclosed in U.S. Pat. No. 4,704,876 to Hill, the subject matter of which is hereby incorporated by reference. In the system of the Hill patent, the railcar walls are insulated, and a ceiling between the railcar roof and floor separates an upper refrigerant compartment or bunker from the lower cargo compartment. The ceiling of the cargo compartment or bunker floor is provided with openings. A distribution pipe coupled to a source of liquid carbon dioxide extends along the length of the upper bunker. The distribution pipe converts liquid carbon dioxide to carbon dioxide snow which is stored in the bunker. During transit, the carbon dioxide snow sublimates and the gas escapes through the openings in the bunker floor to pass into and to cool the cargo in the cargo compartment. The floor of the cargo compartment is provided with open top, lengthwise channels which receive the carbon dioxide and conduct it to one end of the car where it is discharged out an exterior vent in one end wall of the railcar.
The railcar according the Hill patent has been successfully used for transporting frozen products which can be directly exposed to the carbon dioxide gas without adverse effects. However, such system cannot be used with products which cannot be frozen or cannot be exposed to direct contact with the carbon dioxide gas.
Cargo requiring refrigeration at non-frozen temperatures and sensitive to carbon dioxide gas is typically transported in mechanical refrigeration systems employing a circulating refrigerant and a power-driven compressor. Such mechanical systems are disadvantageous due to the need for extensive maintenance.
Carbon dioxide refrigeration systems have been proposed for non-frozen goods, as well as frozen goods. U.S. Pat. No. 4,761,969 discloses a carbon dioxide refrigeration system for storing both frozen and non-frozen goods. For frozen goods, liquid carbon dioxide is conveyed to an upper distribution pipe or manifold for depositing snow into a refrigerant compartment or bunker located above the cargo compartment. The bunker floor separates the refrigerant compartment from the cargo compartment and has openings to permit sublimating carbon dioxide gas to pass from the refrigerant compartment into the cargo compartment. For non-frozen goods, liquid carbon dioxide is conveyed to a lower manifold or distribution pipe located within an expandable bladder. The carbon dioxide gas generated in the bladder is vented directly to the outside of the container, and does not pass into the cargo compartment. The expandable bladder, when filled with carbon dioxide snow, acts as a cold plate to cool the load by convection. This system is disadvantageous in view of the need for an expandable bladder which will retain its resiliency, even when subjected to the extremely cold temperatures of the carbon dioxide snow and since inadequate cooling may be provided along the sides and bottom of the container.
Another dual purpose cryogenic system is disclosed in U.S. Pat. No. 5,152,155. This patent discloses a refrigerated railcar wherein carbon dioxide snow is formed and deposited on a bunker floor from a single distribution pipe located within the single refrigerant compartment in the railcar. For frozen goods, carbon dioxide gas is conveyed through serpentine ducts in the railcar walls and through openings in the bunker floor directly into the cargo compartment. For non-frozen goods, the openings in the bunker floor are closed by a switchable valve such that the flash gas and the sublimating carbon dioxide gas can only pass through the serpentine passages in the walls, isolating the carbon dioxide gas from the cargo. This system is disadvantageous due to the difficulty and potential problems in the mechanism for opening and closing the switchable valves in order to convert the railcar from a frozen mode to a non-frozen mode. Additionally, inadequate cooling may be provided in the walls and floor.